Water-absorbing resins are widely used in various fields, including sanitary products, foods, agriculture and forestry industries, and civil engineering. These resins are generally used particularly in sanitary products such as paper diapers and sanitary napkins, taking advantage of their water absorption. Typical examples of such water-absorbing resins used in sanitary products include partially neutralized salts of polyacrylic acid or polymethacrylic acid.
Water-absorbing resins for use in sanitary products such as paper diapers are required to have a high water absorption capacity not only under normal pressure but also under body pressure (i.e. under pressure).
A possible known technical solution to the above problem is to crosslink the surface of water-absorbing resin particles with a crosslinking agent. In such a surface crosslinking method, the surface layers of water-absorbing resin particles containing a carboxylic acid group and/or a carboxylate group are crosslinked by a crosslinking agent while the water-absorbing resin particles are inhibited from being internally crosslinked in order to maintain the water absorption capacity, whereby a water-absorbing agent having a high water absorption rate can be obtained.
Conventionally, epoxy group-containing compounds have been used as such crosslinking agents. However, the crosslinking agents containing epoxy group-containing compounds may cause problems such as skin irritation. For this reason, crosslinking agents free of epoxy group-containing compounds have been proposed. Examples of these crosslinking agents include those containing compounds having at least two halohydrin groups in their molecule or compounds containing a halohydrin group and a quaternary ammonium group in their molecule (see Patent Literature 1). The examples also include crosslinking agents containing polyhydric alcohols having multiple amide backbones or oxazole backbones in their molecule (see Patent Literature 2). These crosslinking agents also provide water-absorbing resin particles with water absorption capacities comparable to those of the crosslinking agents containing epoxy group-containing compounds.
As described above, while various crosslinking agents that crosslink water-absorbing resin particles have been developed, recent improvements such as reduced thickness of sanitary products (e.g. paper diapers) have created a need for developing water-absorbing agents having higher water absorption capacity.